Southwest Airlines and SeaWorld are calling it quits, opting to mutually end a longtime marketing partnership that dates back to 1988.

Bid farewell to the three whimsical Shamu- and penguin-branded planes that will return to the traditional Southwest design.

The announcement comes following protests earlier this year that sought to persuade the airline to terminate its marketing relationship with SeaWorld in the wake of the documentary "Blackfish" that is critical of the theme park's treatment of killer whales.

Southwest and SeaWorld, which has marine parks in San Diego, Orlando and San Antonio, insist that the decision was a mutual one, predicated on "shifting priorities" by both companies.

Both issued the same statement:

"Southwest and SeaWorld have mutually decided not to renew their partnership when the contract expires at the end of the year," they said. "Our promotional marketing relationship began in 1988 and was one of the first of its kind – focused on co-marketing opportunities between Southwest passengers and SeaWorld visitors.

"Southwest is spreading its wings with new international service, and increased focus on local market efforts. With an increasing international visitor base, SeaWorld is looking to focus on new and growing markets in Latin America and Asia, among others."

The companies will continue to work together through Southwest Vacations but the three branded airplanes will no longer exist.

It was only a year ago that Southwest, in celebration of its then 25-yearpartnership with SeaWorld, debuted its newest specialty aircraft, Penguin One -- a Boeing 737-700 co-branded with images depicting a SeaWorld penguin.

At the time Southwest CEO Gary Kelly said of the new plane, which was filled with Southwest and SeaWorld employees:

"SeaWorld has been an important partner to Southwest Airlines for 25 years. Penguin One is a natural addition to the Southwest fleet and a great representation of the evolution of our partnership and our mutual dedication to travel, tourism, and family entertainment."

People for the Ethical Treatment of Animals (PETA), which has been protesting SeaWorld's treatment of marine mammals for years, cheered the decision. PETA has been pressing Southwest since 2011 to terminate its marketing partnership with SeaWorld.

"Champagne corks were popping at PETA today when we heard that Southwest will no longer support SeaWorld and will repaint its planes," PETA President Ingrid E. Newkirk said. "...The second I heard the good news, I knew that I'd be booking my next trip on Southwest—and kind people around the world no doubt feel the same."

Southwest spokeswoman Marlee McGinness said the decision to part ways was not influenced by pressure from activists but had more to do with the company's new focus on international service, less established local markets like Denver and a commitment to community revitalization efforts.

Earlier this year, when animal rights activists went to Southwest's Dallas headquarters to urge the airline to end its relationship with SeaWorld, Southwest at that time said, "At this time, our partnership with SeaWorld will continue ... we are in a listening and education mode with the goal of upholding our commitments as a good corporate citizen."

The SeaWorld marine parks continue to be dogged by criticism of the use of killer whales in its Shamu shows, and last year the San Diego and Orlando parks experienced a 3 percent and 5 percent decline, respectively, in attendance, according to a theme park report. Higher ticket prices were blamed, in part for the decline.